Power supply for electrostatic spray painting



L. J. LAMM April 17, 1956 POWER SUPPLY FOR ELECTROSTATIC SPRAY PAINTING Original Filed Dec. 1, 1948 lnnenlor Ciltorncgs United rates POWER SUPPLY FOR ELECTROSTATIC SPRAY AIN TIN G Lewis J. Lamm, Washington, D. C., assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware 10 Claims. (Cl. 321-2) The present invention relates to spray coating and more particularly relates to a power supply for spray painting in which an electrostatic field is used to influence the deposition of coating materials.

Due to the necessity of using a very high voltage to produce the necessary electrostatic field, certain difficulties have heretofore been experienced regarding fire hazards and personnel safety. Most of the prior apparatus has utilized a discharge electrode maintained at high potential, to permit electrostatic coating while maintaining the article to be coated at ground potential. Modern continuous conveyor type painting apparatus makes it desirable that the conveyor be maintained at ground potential in order that there will be no requirement of transferring articles from one conveyor to another during the painting and baking operations and in order that the personnel involved in these operations may be protected from contact with conductors carrying high potential electricity. It is however quite desirable, for maximum efiiciency in painting, that the articles being coated be maintained at high potential. Thus two articles may be painted simultaneously, one article acting as a discharge electrode for the other and vice-versa.

It is therefore an object of the present invention to provide an improved power supply for maintaining articles to be coated at high electrical potential during the coating operation.

It is a further object of the present invention to provide means for disabling the high potential circuit, or materially reducing the output thereof, under sparking conditions.

It is a further object of the present invention to anticipate sparking conditions and disable the high potential circuit in response to conditions which may cause sparkmg.

Other objects of this invention will become apparent upon reading the specification and inspection of the drawings and will be particularly pointed out in the claims.

Referring to the drawings, Figure l is a circuit diagram showing one form of power supply embodying the invention and arranged to charge the articles to be coated with a negative potential.

Figure 2 is an alternate circuit showing the circuit of Figure 1 modified to charge the articles at a positive potential.

This is a division of my application Serial No. 62,913, filed December 1, 1948, now Patent No. 2,626,589, issued January 27, 1953, to which reference may be had for examples of electrostatic spray painting apparatus with which the present invention may be used, without, hoW ever, limiting it to that use.

Referring now to Figure 1, a novel circuit incorporating the novel features of the present invention is illustrated. It is well-known in the electrical art that the loading of the secondary of a double tuned circuit reflects resistance back into the primary causing loading of that circuit. It is also well-known that in certain types of oscillators the loading of the plate circuit will cause the oscillator to go atent ice out of oscillation. These two phenomena are utilized in Figures 1 and 2 to provide a circuit which anticipates sparking conditions and operates to deenergize the circuit in response thereto. The secondary circuit will load under arcing or arc anticipating conditions or under conditions where there is a flame ionizing the electrostatic field sufficiently to load the primary circuit and thus deenergize the circuit.

Referring now particularly to Figure 1, an alternating current 141 from a standard 110, 220 or 440 volt power source is introduced across the transformer primary 82. This primary is magnetically coupled to a center tapped secondary 84 capable of producing an A. C. plate voltage for the tubes 86 and 88. These tubes combine to produce a push-pull oscillator with a common output tank circuit consisting of condenser 90 and inductance 92. The inductance 92 is tapped to ground. One end of the inductance 92 is connected to the grids of the tubes 86 and 88 to produce self oscillation. The plates of these two tubes are energized respectively by alternate half cycles of the power supply voltage. The plates of the tubes 86 and 88 are both connected to the other end of the inductance 92 through condensers 94 and 96 respectively. It may thus be seen that the tuned circuit including the condenser 90 and the inductance 92 is thereby kept in continuous oscillation. The condenser 98 and resistance 100 provide a selfbiasing circuit for the grids of these two tubes. The coils 102 and 104 provide a certain amount of filter action, while the coil 104 in addition to providing filter action, also provides actuation for the overload relay 106. The electrical characteristics of coils 102 and 104 are similar. The tank circuit, consisting of condenser 90 and inductance 92, has a high natural resonant frequency in the order of magnitude of several kilocycles. The inductance 92 is magnetically coupled to the coil 108, which together with the condenser 110, has the same resonant frequency as the tank circuit previously referred to. The coils 92 and 108 are so wound that their mutual inductance provides approximately critical coupling. A rectifier 112 is connected to the high voltage side of the resonant circuit including the secondary inductance 108 and its tuning condenser 110. It may thus be readily seen that the high voltage high frequency current produced across the secondary tuned circuit in Figure 1, is rectified so that a high voltage D. C. is produced on the electrode 31.

Under normal operating conditions, the D. C. drain on the electrode 31 is very small and in the order of a few milliamperes. However, under short circuiting conditions, the current drain on this electrode is limited only by the power output of the power supply and may easily reach proportions sufiicient to cause sparking. In the present invention the loading of the secondary tuned circuit by such short circuiting reflects aresistance back into the tuned circuit 90-92 thus causing the tank circuit to be heavily loaded and to stop or sharply reduce oscillation of the circuit. Under these conditions the tubes 86 and 88 have very high current drain thus causing sufficient current to pass through the coil 104 to actuate the manually reset relay 106. This deenergizes the oscillator circuit and thus quenches the arc formed at the short circuit.

Referring to Figure 2, the rectifier 116 is identical to the rectifier 112 in Figure 1 except that its polarity is reversed. The rest of the circuit including electrode 35, is identical with that shown in Figure 1. The circuits shown in Figures 1 and 2 will of course be actuated in response to current flow caused by fire ionizing the gas in the electrostatic field as set forth in my application Serial No. 62,913 and may also be similarly used to control the introduction of fire extinguishing gas into the spray booth.

While this description covers a specific embodiment of the present invention, it is not intended that the invention shouldbe limited to this specific embodiment but that it should be construed as bread as possible in conformance-with the hereto appended claims. i

I claim:

1. A direct current power supply including; an alternating current source, an electronic oscillatorzhaving a tuned circuit and an electronic tube, said oscillator: being of such capacity as to cease the generation of oscillatory currents upon excessive loading of said tuned circuit, a circuit for connecting said alternating current source to the plate of. said oscillator tube, relay means located in said last mentioned circuit, said relay means operable on the drawing of high plate current to deenergize said oscillator, and a rectifier circuit coupled to said tuned'circuit and responsive on the drawing of excess current in said rectifiercircuit to loadsaid tuned'circuit thus'c'ausing' cessation of the generation 'ofoscillatory currents with-resultant flow of high plate current and the 'ac'tuation of said relay to deenergize said oscillator;

2. A direct current power supply including; an alterriating current source, an electronic oscillator-having a tuned circuit and an electronic tube, said oscillator being of such capacity as to sharply reduce the generation of oscillatory currents upon excessive loading of said tuned circuit, a plate circuit for connecting said alternating current source to said oscillator, relay means located in said last mentioned circuit, said relay means operable on the drawing of plate current to substantially reduce the plate power to and the output of said oscillator, a rectifier circuit connected to said tuned circuit and responsive on the drawing of'excess current in said rectifier circuit to load said tuned circuit, thus causing substantial reduction of the oscillator current with resultant fiow of plate current and the actuation of said relay to reduce the power supplied to said oscillator.

3. A direct current power supply including; an alternating current source, an electronicv oscillator havinga tuned 'circuit and a plurality of electronic tubes, said oscillator being of such capacity as to sharply reduce the generation of oscillatory currents upon excessive loading of said tuned circuit, a circuit for connecting said alternating current source to the plates of said oscillator tubes, relay means located in the plate circuit'of-one 'of-the oscillator tubes, said relay means operable on the drawing of excessive plate current to deenergize said one oscillator'tube, and a rectifier coupled to said tuned 'circuit and responsive on the drawing of excess current in said rectifiercircuit thus causing reduction of'oscillatory currents to load said tuned circuit with resultant How of excessive plate current and'the actuation of said relay to deenergize said one of said oscillator tubes. 4. A direct current power supply circuit comprising, an alternating current source, a pair of electron tubes, ai'circuit supplying alternating current from said source tothe plates of said tubes, a relay having its winding and contacts in series with the plate of one of said tubes, a'winding in said circuit, said winding having'e'lectrical characteristics similar to those of said relay winding, tuned circuit means for causing said tubes to' function as an electronic oscillator having a capacity such that it sharply reduces the generation of oscillatory currents upon excessive loading of said tuned circuit, means controlled by said relay for varying the supply of alternating current to said tubes, and a rectifier circuit connected to said tuned circuit and responsive on the drawing of excess current to load said tuned circuit and to actuate said relay and cause reduction of the oscillator current. 5. A directcurrent power supply comprising an alternating current source, an electronic oscillator having a plurality of tubes, a transformer having a primary winding connected to said source and a secondarywinding having a plurality of circuits, means'for connectingeach of said secondary circuits to the plate of one of said tubes, a-relay in one of said plate circuits for controlling the power output of said transformer, a tuned circuit in the output of said oscillator, a second tuned circuit coupled to said tuned output circuit, and a rectifier connected to said second tuned circuit and responsive on the drawing of excess current in said rectifier circuit to load said tuned circuits to actuate said relay and reduce the fiow of oscillator current.

6. A direct current power supply of the character described, including: an oscillator having an input circuit and an output circuit, said oscillator having the characteristic of ceasing to generate oscillatory currents upon excessive loading of said output circuit; electrically operated switch means connected in series '-with the input circuit of said oscillator, said switch means being operable upon excessive current flow in said input circuit; and a rectifier circuit coupled to said output circuit and operable, upon flow of excessive current therethrough, to load said output circuit and cause cessation of the generation of oscillatory currents with resultant excessive current flow in said input circuit and consequent operation of said switch means to deenergize said oscillator.

7. A direct current power supply of the character described, including: an oscillator having an input-circuit and a tuned output circuit, said oscillator having the characteristic of ceasing to generate oscillatory currents upon excessive loading of said tuned circuit; electro-magnetic switch means connected to the input circuit of said oscillator, said switch means being operable upon excessive current flow in said input circuit; and a rectifier circuit coupled to said tuned circuit and operable, upon How of excessive current therethrough, to load said tuned circuit and cause cessation of the generation of oscillatory-currents with resultant excessive current flow in said input circuit and consequent operation of said switch means to deenergize said oscillator.

. 8. A direct current power supply of the character described, including: an oscillator having an input circuit and a tuned output circuit, said oscillator having the characteristic of sharply reducing the generation of oscillatory currents upon excessive loading of said tuned circuit; a relay connected in series'with the input circuit of said oscillator, said relay being operable upon high current flow in said input circuit; and a rectifier circuit coupled to said tuned circuit and operable, upon fiow of high current therethrough, to load said tuned circuit excessively and cause a sharp reduction in the generation of oscillatory currents with resultant high current flow in said input circuit and consequent operation of said relay to de-energize said oscillator.

9. A-direct current power supply of the character dc scribed including: a source of power; an oscillator connected in circuit with said power source for energization therefrom and having a tuned output circuit, said oscillator having the characteristic of ceasing to generate oscillatory currents upon excessive loading of said-outputcircuit; electrically operated switch means connected in the energizing circuit of said oscillator and being operable upon excessive current flow drawn by said oscillator; and a rectifier circuit coupled to said tuned output circuit and operable upon flow of excessive current therethrough to load said output circuit and cause cessation of the'generation of oscillatory currents with resultant excessive current flow in said energizing circuit and consequent operation of said switch means to de-energize said oscillator.

10. A direct current power supply of the character described, including: a source of power, an oscillator .connected in circuit with said power source for energization therefrom and having a tuned output circuit, said oscillator including a plate, grid and cathode electrodeand a self biasing circuit including a parallel connected condenser and resistor. connected in a circuit containing the plateand. grid electrodes of the oscillator, saidoscillator having the characteristic of ceasing to generate oscillatory currents upon excessive loading of said output circuit; electricallyoperated switchimeans connected in the ener gizing circuit of said oscillator and being operable upon excessive current flow drawn by said oscillator; and a rectifier circuit coupled to said tuned output circuit and operable upon flow of excessive current therethrough to load said output circuit and cause cessation of the 5 generation of oscillatory currents with resultant excessive current fiow in said energizing circuit and consequent operation of said switch means to de-energize said oscillator.

References Cited in the file of this patent UNITED STATES PATENTS 

